1. Field of the Invention
The present invention relates generally to a voltage converter and more specifically to switched low voltage converters that can operate when provided with an input voltage that is less than the threshold voltage of the switching devices used in the voltage converter.
2. Description of the Related Art
In today's electrical devices and equipment, voltage converters are widely used to convert an output voltage of a single power source, such as a battery, to a few voltage rails that, in turn, power different functional blocks within a system.
One example is a mobile handset powered by a single cell Li-Ion battery with a 3.6V nominal output voltage. Within the mobile handset, there are usually 3 to 5 voltage rails, such as 5V, 3.3V, 2.5V, 1.8V, and 1.5V, etc., that are needed to power different functional blocks within the handset. All these voltage rails are converted from the Li-Ion battery. Because some of the voltages, such as 5V, are higher than the nominal output voltage of the single-cell Li-Ion battery, voltage converters with a step-up or “boost” topology, are used.
A boost converter is a switching mode converter that typically contains a diode and an energy storage element. An example of a conventional boost converter is shown in FIG. 1. The boost converter 10 includes a high voltage power source (HV_BAT) 12, an energy storage element in the form of an inductor (L1) 14 connected in series with the HV_BAT, a switching device (SW_1) 16 connected between the inductor and ground (GND), a diode (D1) 17 connected in series with (L1), a capacitor (C1) 18 connected between D1 and GND, and a voltage divider, a first resistor (R1) 20 and a second resistor (R2) 22, that is also connected between D1 and GND. The voltage across the voltage divider is the output voltage (VOUT) of the voltage converter that is provided to a load (RL) 24. The voltage at the connection between R1 and R2 provides an input voltage to a Pulse Width Modulator (PWM) 26 that provides an input signal (PWM_SW) to the control terminal of switching device (SW_1). The PWM obtains power from terminals that are connected between D1 and GND.
When the power source, HV_BAT is first attached to the converter, a voltage VD appears between D1 and the load (RL). This “first” VD serves two purposes: it provides an intermediate “first push” voltage that is required to activate the internal PWM block, and it provides the “initial” output voltage, VOUT, to the load (RL). A constant VOUT is achieved by using the PWM to switch the switching device SW_1 on and off to control the amount of energy stored in L1 and C1. The PWM block uses the voltage at the junction between R1 and R2 (VFB) to determine the duty cycle of the PWM output signal. When the desired voltage (determined by the values of R1 and R2) is obtained, the duty cycle of the PWM stabilizes and the output of the boost converter is a constant output voltage at the desired level.
For a boost converter to operate, the initial output of the PWM must be at a sufficient level (VTH) to switch SW_1 ON. VTH is defined by the nature of the switch. After the SW_1's “first switch on,” the boost converter starts operation normally, and its output voltage VOUT eventually reaches a constant voltage required by the load RL. If VTH is insufficient, the SW_1 will not be switched on the first time, as a result, the boost converter illustrated in FIG. 1 will not start. In a typical semiconductor based boost converter, VTH is between 0.7V and 1.0V, depending upon the technology of the switching device (MOSFET, bipolar transistor, etc.) used in the voltage converter. This means the power source has to have an output of at least 0.7V to 1.0V.
Renewable power sources can be used to replace environmentally hazardous chemical and electrochemical based energy sources, such as a variety of Li-Ion, Li-Polymer, NiMH, and NiCd batteries, which are still being widely used in mobile handsets and other battery-powered consumer and commercial devices and equipment as the primary source of energy. A promising energy source is the solar cell. However, a typical single solar cell outputs a voltage that does not exceed 0.3V, which is significantly below the 0.7V to 1.0V VTH of a typically switching element in conventional voltage converters.
Research is underway to develop semiconductor processing technologies on which ultra low threshold switching devices (i.e. switching devices with thresholds below 0.3V) can be made. However, even if such new semiconductor processing technologies can indeed be developed, the lower the threshold voltage becomes, the more complicated, and therefore more costly the processing technology is likely to become. In addition, no matter how low the threshold voltage of the switching devices becomes, conventional topologies cannot achieve a voltage converter that has a zero voltage threshold.